Experimental models of pneumonia after intratracheal installation of L. pneumophila have been developed in guinea pigs and rats. Attempts to reproduce these models using an infectious aerosol to which only the snouts of animals are exposed will be undertaken. If the differences in susceptibility to an infection already observed are reproduced by the aerosol challenge, these models will be contrasted in detail. In particular, killing and clearance of inhaled 3H-label L. pneumophila will be measured at various times after exposure. Alterations in capillary permeability of infected animals will be studied by appropriate electron micrographic techniques. Alterations in susceptibility to infection of the experimental animals will be induced by prior treatment with corticosteroids, immunosuppression with pharmacologic or immunologic intervention, or immunity will be enhanced by prior exposure to sub-lethal concentrations of bacteria.